CONTRIBUTIONS TO PALEONTOLOGY. 179 



AGNOSTUS PAEILIS (n. s.). 



PLATE X. FIGS. 24 & 25. 



The two extremities of similar form. The head semielliptical ; 

 length and width about equal, very convex in the posterior 

 part, and curving downwards to the anterior margin. 



The central portion of the posterior part is limited by a faint 

 curving groove ; and anterior to its limit there is a slight elevation, 

 which may have been a node on the surface of the crust. The 

 posterior margin, just within the angles, is produced in a minute 

 node. The marginal rim gradually expands from the posterior 

 angles to the front, where it becomes well defined. 



The pygidium is of the same form as the head, slightly truncate 

 at the anterior angles, and the marginal rim narrower towards 

 the articulating border : the central part is slightly more eleva- 

 ted, and limited by furrows diverging from the anterior margin. 



In the median line, at a point one-third the length from the 

 anterior margin, there is a distinct elongate node. 



Fig. 24. The head, about three times enlarged. 



Fig. 25. The pygidium, enlarged in the same degree. 



This species occurs in a yellowish or light drab-colored sandstone, on the 

 shores of Lake Pepin. It is a well-marked species, and readily distinguished 

 from the others by the shallow grooves on both parts, and the distinct node 

 upon the pygidium. 



In the part which I have termed the head, there is some appearance of 

 an elongate glabella in front of the curving groove, but the indications are 

 too faint to be satisfactory. 



This species, and the Agnostus disparilis, occur in beds near the middle 

 of the formation; while A.josepha occurs in beds which are apparently 

 somewhat lower in the series, but the precise relations have not been 

 determined. 



AGNOSTUS DISPARILIS (n.s.). 



PLATE X. FIGS. 25, 26 & 27. 



Head semielliptical, convex towards the posterior side and ab- 

 ruptly sloping to the front ; length and breadth nearly as three 

 to four; a little concave on the posterior or articulating margin, 

 and marked near the edge by a narrow groove on each side of 

 the middle, the centre a little elevated close to the margin. 

 The marginal rim is a little wider in front than at the sides. 



